1. Field of the Invention
The present invention generally relates to a wafer structure and a bumping process, in particular, to a wafer structure and a bumping process for releasing the pressure exerted on bumps.
2. Description of Related Art
Recently, the flat display device having advantages of higher image quality, optimal space efficiency, low power and non-radiation is broadly used in medium or small-sized portable televisions, cell phones, camcorders, notebook computers, desktop computers, projection-type televisions and other computer products. Therefore, the flat display device has replaced the conventional cathode ray tube (CRT) and become the main stream on the market. In general, after the fabrication process of the flat display device is completed, there is a need to perform a chip on glass (COG) process, such that the flat display device can be electrically connected to external systems though some chips, to provide power or driving signals to the flat display device.
FIG. 1 is a schematic cross-sectional view showing a flat display device and a chip when performing a COG process. Please refer to FIG. 1, a chip 200 is disposed on a flat display device 100, wherein the bumps 202 of the chip 200 are aligned with the bonding pads 102 of the flat display device 100. After that, the chip 200 and the flat display device 100 are heated and pressed, so as to bond the bumps 202 and the bonding pads 102 together. However, too much bonding force applied to the chip 200 and the flat display device 100 during the COG process may damage the chip 200, the flat display 100 and the electrical connection between them. Therefore, a compliant bump is provided to solve the foregoing problem.
FIG. 2 is a schematic cross-sectional view showing a conventional compliant bump. Please refer to FIG. 2, a substrate 300 comprises a plurality of pads 310 disposed on its active surface S and a solder mask layer 320 exposing a portion of the pads 310. A plurality of under bump metallurgy (UBM) layers 330 are disposed on the solder mask layer 320 and contact the exposed pads 310 respectively; a plurality of compliant bumps 340 are disposed on the UBM layers 330 respectively.
Each compliant bump 340 comprises a polymer material 340a and a metal shell 340b enclosing the polymer material 340a. In general, the polymer material 340a is polyimide. However, if water or solvent remains in the polymer material 340a enclosed by the metal shell 340b after the fabrication process of the compliant bumps 340 is completed, the compliant bumps 340 may expand, crack or deform when heated. This may damage the electrical connection between the substrate 300 and other devices.